A common technique for speech coding is the so-called LPC coding in which at a coder, an input speech signal is divided into time intervals and each interval is analysed to determine the parameters of a synthesis filter whose response is representative of the frequency spectrum of the signal during that interval. The parameters are transmitted to a decoder where theiy periodically update the parameters of a synthesis filter which, when fed with a suitable excitation signal, produces a synthetic speech output which approximates the original input.
Clearly the coder has also to transmit to the decoder information as to the nature of the excitation which is to be employed. A number of options have been proposed for achieving this, falling into two main categories, viz.
(i) Residual excited linear predictive coding (RELP) where the input signal is passed through a filter which is the inverse of the synthesis filter to produce a residual signal which can be quantised and sent (possibly after filtering) to be used as the excitation, or may be analysed, e.g. to obtain voicing and pithc parameters for transmission to an excitation generator in the decoder.
(ii) Analysis by synthesis methods in which an excitation is derived such that, when passed through the synthesis filter, the difference between the output obtained and the input speech is minimised. In this category there are two distinct approaches: One is multipulse excitation (MP-LPC) in which a time frame corresponding to a number of speech samples contains a, somewhat smaller, limited number of excitation pulses whose amplitudes and positions are coded. The other approach is stochastic coding or coded excited linear prediction (CELP). The coder and decoder each have a stored list of standard frames of excitations. For each frame of speech, that one of the codebook entries which, when passed through the synthesis filter, produces synthetic speech closet to the actual speech is identified and a codeword assigned to it is sent to the decoder which can then retrieve the same entry from its stored list. Such codebooks may compiled using random sequence generation; however another variant is the so-called `sparse vector ` codebook in which a frame contains only a small number of pulses (e.g. 4 or 5 pulses out of 32 possible positions with a frame). A CELP coder may typically have a 1024-entry codebook.
The present invention is defined in the appended claims.
Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: